As taught in my copending application Ser. No. 244,402, improved results can be achieved with respect to recovering hydrogen chloride from any organochlorosilane hydrolysis mixture by using a substantially stoichiometric equivalence of water to hydrolyze the organochlorosilane. Further benefits can be obtained if the aqueous hydrolysis product free of the organopolysiloxane hydrolyzate is recycled as the aqueous hydrolysis medium.
Experience has shown, however, that if the aqueous water feed is continuously recycled after it has been separated from the organopolysiloxane hydrolyzate and reused as the source of water which automatically increases the acidity of the hydrolysis medium, the chloride content, i.e. chemically combined or dissolved chloride, of the resulting organopolysiloxane hydrolyzate also substantially increases. In addition, the yields of cyclic organopolysiloxanes are reduced. The buildup of --SiCl end groups and dissolved HCl result in loss of organopolysiloxane hydrolyzate and chlorine in the form of HCl and requires additional process steps to minimize such losses. For example, neutralization of dissolved or chemically combined chloride in the organopolysiloxane hydrolyzate to reduce its acidity results in a loss of chloride values.
As used hereinafter, the expression "WT% total chloride" means total titrated chloride in the organopolysiloxane hydrolyzate either as dissolved HCl or as chloride attached to silicon as terminal --SiCl groups.
The present invention is based on my discovery that if organohalosilanes having the formula, EQU R.sub.(a) H.sub.(b) SiX.sub.(4-a-b), (1)
where R is selected from a C.sub.(1-13) monovalent hydrocarbon radical and substituted monovalent hydrocarbon radical, X is a halogen radical, for example chloro, a is an integer equal to 1 to 3 inclusive, b is a whole number equal to 0 to 2 inclusive and the sum of a+b is equal to 3, is hydrolyzed with from about 10 to about 30 moles of water in the form of a concentrated or saturated HX solution at a temperature of from about 0.degree. C. to 60.degree. C., organopolysiloxane hydrolyzate having a higher cyclic siloxane content and also having a reduced weight percent of total halide, based on the weight of organopolysiloxane hydrolyzate can be recovered. For example, within a residence time of 5 minutes in the hydrolysis reactor, the WT% of total chloride obtained by the practice of the method of the present invention will be approximately 1/2 of the value obtained at the same residence time using substantially stoichiometric amounts of water and organochlorosilane of formula (1).
In addition hydrogen chloride can be recovered directly from the mixture without any requirement of distillation of the aqueous hydrogen chloride solution.
Radicals included within R are, for example, methyl, ethyl, propyl, phenyl, etc., which can be the same or different when a is greater than one.